As is known, a passive mixer is often used in a linear RF receiver system to meet stringent linearity and noise requirements. An example of a passive mixer is a double balanced resistive Field Effect Transistor (FET) mixer, where all the ports are operated differentially. In the FET mixer, local oscillator (LO) signals are applied to the gates, RF signals are applied to sources (or drains) and intermediate frequency (IF) signals are taken from the drains (or sources). In order to achieve the high linearity and low noise requirements, a relatively large LO signal is required, which LO signal is typically greater than 10 dBm.
The resistive FET mixer works by changing the resistance between gate to source from low to high resistance. The high intercept point (IP3) of the FET mixer is achieved by operating the FETs of the FET mixer at a linear region and completely switching the gates on and off. Therefore, it is important to properly bias the FETs of the FET mixer by apply a relatively large LO signal to the FET mixer. The large LO signal applied to the FET mixer is also important for achieving a low conversion loss and thus a low noise figure. A typical LO signal level that is required to achieve an IP3 of greater than 15 dBm is over 10 dBm. In order to deliver 10 dBm of power in a 50 ohm system, for example, a peak-to-peak current of 40 mAp-p or equivalently peak-to-peak voltage of 2 Vp-p is required. Accordingly, the LO signal should be amplified from 0 dBm, which is the typical output power level of the LO, to 10 dBm for properly driving the FET mixer. In order to amplify the LO signal from 0 dBm to 10 dBm, however, requires that the LO signal be passed through a linear amplifier prior to being provided to the FET mixer.
One drawback or disadvantage to using a linear amplifier to boost the power level of the LO signal from 0 dBm to 10 dBm and to provide the minimum peak-to-peak current of 40 mAp-p is the excessive power required by the linear amplifier itself. In this example, a minimum of 20 mA of bias current is needed to operate the linear amplifier. This power required to operate the linear amplifier (e.g., 20 mA) is a substantial portion of the overall power dissipation of a typical RF receiver.
It would, therefore, be desirable to overcome the aforesaid and other disadvantages.